Among the various antiresonant suspension devices employed at the present time, one of the most widely used is that having three degrees of freedom and possessing the characteristics disclosed in U.S. Pat. Nos. 4,431,148 and 4,458,862 of the applicant.
Such devices are applicable to helicopters comprising a fuselage, a driving unit, a lifting and propelling rotor and a transmission box which acts as a speed reducer located between said driving unit and said rotor and in alignment with the axis of the latter. They mainly comprise a set of at least three support members the upper ends of which are connected to the upper housing of the transmission box whereas the lower ends are articulated to the outer ends of arms arranged radially about the base of the transmission box, to which base they are connected, these arms also being connected in the region of their outer ends to the fuselage at strong points of the latter.
Each radial arm comprises a deformable portion and the various connections of each radial arm are so disposed that they allow it movements by elastic deformation in a radial plane containing the axis of the rotor and the arm. The movement of each radial arm imparts large displacements to a beating weight mounted at the end of a rigid support whose other end is connected to the end of the corresponding axial arm.
Said suspension devices further comprise a suspension plate substantially contained in a plane perpendicular to the axis of the rotor. The central portion of the suspension plate is connected to the bottom of the transmission box and its periphery is connected to the structure of the upper portion of the fuselage of the helicopter. This plate may be formed by a diaphragm which is flexible in bending so as to allow the bottom of the transmission box:
limited alternating movements in vertical translation (pumping), and/or
limited alternating movements of oscillation (roll and pitch) about axes contained in the plane of the diaphragm.
The diaphragm is sufficiently rigid in its plane to transmit without substantial deformation the reaction of the driving torque of the helicopter on the transmission box and fuselage.
This plate may also be formed by a horizontal beam the ends of each of the arms of which are connected to the structure of the fuselage by two articulated links operating under tension for transmitting to the transmission box and the fuselage the reaction of the driving torque of the helicopter while allowing small vertical displacements and angular oscillations of the transmission box about axes contained in the plane of the plate.
In these devices, the spring/antiresonant beating weight mechanism interposed between the lower ends of the members supporting the transmission box on the fuselage and the strong attachment points on the top of the fuselage, is arranged horizontally in a plane perpendicular to the axis of the rotor, so that the oscillations of the beating weights produced by the deformations of the flexible portion of the arms occur in vertical planes and the inertial forces generated on the supports of said beating weights therefore have solely a vertical direction.
Although these suspension devices operate in a quite satisfactory manner and it is easy to find compromises for, on one hand, ensuring through said suspensions the transmission between the rotor and the fuselage of static forces and moments which act on the rotor head of the helicopter, and, on the other hand, for obtaining an effective filtering of the dynamic components of the forces and moments which have an effect on said rotor head, and in particular the component of the force perpendicular to the plane of the rotor and the vibrations resulting from the forces and moments in the plane of the rotor, the applicant has found that it is possible to improve them and thereby improve the performances.